| 1. |
- Baldissera, Gustavo, et al.
(författare)
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Understanding the optical properties of ZnO1-xSx and ZnO1-xSex alloys
- 2016
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 119:4
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Tidskriftsartikel (refereegranskat)abstract
- ZnO1-xYx with chalcogen element Y exhibits intriguing optoelectronic properties as the alloying strongly impacts the band-gap energy E-g(x). In this work, we analyze and compare the electronic structures and the dielectric responses of Zn(O,S) and Zn(O, Se) alloys by means of the density functional theory and the partially self-consistent GW approach. We model the crystalline stability from the total energies, and the results indicate that Zn(O, S) is more stable as alloy than Zn(O, Se). We demonstrate also that ion relaxation strongly affects total energies, and that the band-gap bowing depends primarily on local relaxation of the bonds. Moreover, we show that the composition dependent band-gap needs to be analyzed by the band anti-crossing model for small alloying concentration, while the alloying band-bowing model is accurate for strong alloying. We find that the Se-based alloys have a stronger change in the band-gap energy (for instance, Delta E-g(0.50) = E-g(ZnO) -(E)g(x = 0.50) approximate to 2.2 eV) compared with that of the S-based alloy (Delta E-g(0.50) = 1.2 eV), mainly due to a stronger relaxation of the Zn-anion bonds that affects the electronic structure near the band edges. The optical properties of the alloys are discussed in terms of the complex dielectric function epsilon(omega) = epsilon(1)(omega) + i epsilon(2)(omega) and the absorption coefficient alpha(omega). While the large band-gap bowing directly impacts the low-energy absorption spectra, the high-frequency dielectric constant epsilon(infinity) is correlated to the intensity of the dielectric response at energies above 4 eV. Therefore, the dielectric constant is only weakly affected by the non-linear band-gap variation. Despite strong structural relaxation, the high absorption coefficients of the alloys demonstrate that the alloys have well-behaved optoelectronic properties.
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| 2. |
- Bostrom, Mathias, et al.
(författare)
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Lifshitz interaction can promote ice growth at water-silica interfaces
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 95:15
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Tidskriftsartikel (refereegranskat)abstract
- At air-water interfaces, the Lifshitz interaction by itself does not promote ice growth. On the contrary, we find that the Lifshitz force promotes the growth of an ice film, up to 1-8 nm thickness, near silica-water interfaces at the triple point of water. This is achieved in a system where the combined effect of the retardation and the zero frequency mode influences the short-range interactions at low temperatures, contrary to common understanding. Cancellation between the positive and negative contributions in the Lifshitz spectral function is reversed in silica with high porosity. Our results provide a model for how water freezes on glass and other surfaces.
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| 3. |
- Bostrom, M., et al.
(författare)
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The influence of Lifshitz forces and gas on premelting of ice within porous materials
- 2016
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Ingår i: Europhysics letters. - : Institute of Physics Publishing (IOPP). - 0295-5075 .- 1286-4854. ; 115:1
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Tidskriftsartikel (refereegranskat)abstract
- Premelting of ice within pores in earth materials is shown to depend on the presence of vapor layers. For thick vapor layers between ice and pore surfaces, a nanosized water sheet can be formed due to repulsive Lifshitz forces. In the absence of vapor layers, ice is inhibited from melting near pore surfaces. In between these limits, we find an enhancement of the water film thickness in silica and alumina pores. In the presence of metallic surface patches in the pore, the Lifshitz forces can dramatically widen the water film thickness, with potential complete melting of the ice surface.
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| 4. |
- Boström, Mathias, et al.
(författare)
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Increased porosity turns desorption to adsorption for gas bubbles near water-SiO2 interface
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:7
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Tidskriftsartikel (refereegranskat)abstract
- We consider theoretically the retarded van der Waals interaction of a small gas bubble in water with a porous SiO2 surface. We predict a possible transition from repulsion to attraction as the surface is made more porous. It highlights that bubbles will interact differently with surface regions with different porosity (i.e., with different optical properties).
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| 5. |
- Chen, Rongzhen, et al.
(författare)
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Electronic and optical properties of Cu2 X SnS4 (X = Be, Mg, Ca, Mn, Fe, and Ni) and the impact of native defect pairs
- 2017
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 121:20
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Tidskriftsartikel (refereegranskat)abstract
- Reducing or controlling cation disorder in Cu2ZnSnS4 is a major challenge, mainly due to low formation energies of the anti-site pair (Cu Zn - + Zn Cu +) and the compensated Cu vacancy (V Cu - + Zn Cu +). We study the electronic and optical properties of Cu2XSnS4 (CXTS, with X = Be, Mg, Ca, Mn, Fe, and Ni) and the impact of defect pairs, by employing the first-principles method within the density functional theory. The calculations indicate that these compounds can be grown in either the kesterite or stannite tetragonal phase, except Cu2CaSnS4 which seems to be unstable also in its trigonal phase. In the tetragonal phase, all six compounds have rather similar electronic band structures, suitable band-gap energies Eg for photovoltaic applications, as well as good absorption coefficients α(ω). However, the formation of the defect pairs (C u X + X Cu) and (V Cu + X Cu) is an issue for these compounds, especially considering the anti-site pair which has formation energy in the order of ∼0.3 eV. The (C u X + X Cu) pair narrows the energy gap by typically ΔEg ≈ 0.1-0.3 eV, but for Cu2NiSnS4, the complex yields localized in-gap states. Due to the low formation energy of (C u X + X Cu), we conclude that it is difficult to avoid disordering from the high concentration of anti-site pairs. The defect concentration in Cu2BeSnS4 is however expected to be significantly lower (as much as ∼104 times at typical device operating temperature) compared to the other compounds, which is partly explained by larger relaxation effects in Cu2BeSnS4 as the two anti-site atoms have different sizes. The disadvantage is that the stronger relaxation has a stronger impact on the band-gap narrowing. Therefore, instead of trying to reduce the anti-site pairs, we suggest that one shall try to compensate (C u X + X Cu) with (V Cu + X Cu) or other defects in order to stabilize the gap energy.
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| 6. |
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| 7. |
- Chen, Rongzhen, et al.
(författare)
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Exploring the electronic and optical properties of Cu2Sn1-xGexS3 and Cu2Sn1-xSixS3 (x = 0, 0.5, and 1)
- 2017
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951.
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Tidskriftsartikel (refereegranskat)abstract
- To accelerate environmental friendly thin-film photovoltaic technologies, earth-abundant, non-toxic, and low-cost materials are demanded. We study the compounds of Cu2Sn1−xGexS3 and Cu2Sn1−xSixS3 (x = 0, 0.5, and 1) employing first-principles method within the density functional theory. The compounds have comparable band dispersions. The band-gap energies Eg can be tailored by cation alloying the Sn atoms with Ge or Si. The gap energies of Cu2Sn1−xGexS3 and Cu2Sn1−xSixS3, with x = 0, 0.5, and 1, vary almost linearly from 0.83 to 1.43 eV and 2.60 eV, respectively. However, the gap energy of Cu2SiS3 does not follow the linear relation for x > 0.8. The effective electron masses at the Γ-point of the lowest conduction band are almost isotropic for all materials, which are between 0.15m0 and 0.25m0. On the other hand, the effective hole masses of the topmost valence band show very strong anisotropy for all compounds. In the (010) direction, the hole masses are estimated to be between 1.01m0 and 1.85m0, while between 0.11m0 and 0.41m0 in the (001) direction. Calculations reveal that all compounds have high absorption coefficients that are comparable with that of Cu2ZnSnS4. The absorptions in the energy region from Eg + 0.5 eV to Eg + 1.0 eV are even higher for Ge- and Si-alloying of Cu2SnS3, compared with Cu2ZnSnS4. The high-frequency dielectric constants of the compounds are between 6.8 and 8.9. Cu2Sn1−xGexS3 and Cu2Sn1−xSixS3 can be considered as potential candidates for absorber materials in thin-film solar cells.
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| 8. |
- Chen, Rongzhen
(författare)
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First-Principles Study on Electronic and Optical Properties of Copper-Based Chalcogenide Photovoltaic Materials
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- To accelerate environmentally friendly thin film photovoltaic (PV) technologies, copper-based chalcogenides are attractive as absorber materials. Chalcopyrite copper indium gallium selenide (CIGS ≡ CuIn1–xGaxSe2) is today a commercially important PV material, and it is also in many aspects a very interesting material from a scientific point of view. Copper zinc tin sulfide selenide (CZTSSe ≡ Cu2ZnSn(S1–xSex)4) is considered as an emerging alternative thin film absorber material. Ternary Cu2SnS3 (CTS) is a potential absorber material, thus its related alloys Cu2Sn1–xGexS3 (CTGS) and Cu2Sn1–xSixS3 (CTSS) are attractive due to the tunable band gap energies. CuSb(Se1–xTex)2 and CuBi(S1–xSex)2 can be potential as ultra-thin (≤ 100 nm) film absorber materials in the future. In the thesis, analyses of these Cu-based chalcogenides are based on first-principles calculations performed by means of the projector augmented wave method and the full-potential linearized augmented plane wave formalisms within the density functional theory as implemented in the VASP and WIEN2k program packages, respectively.The electronic and optical properties of CIGS (x = 0, 0.5, and 1) are studied, where the lowest conduction band (CB) and the three uppermost valence bands (VBs) are parameterized and analyzed in detail. The parameterization demonstrates that the corresponding energy dispersions of the topmost VBs are strongly anisotropic and non-parabolic even very close to the Γ-point. Moreover, the density-of-states and constant energy surfaces are calculated utilizing the parameterization, and the Fermi energy level and the carrier concentration are modeled for p-type CIGS. We conclude that the parameterization is more accurate than the commonly used parabolic approximation. The calculated dielectric function of CuIn0.5Ga0.5Se2 is also compared with measured dielectric function of CuIn0.7Ga0.3Se2 collaborating with experimentalists. We found that the overall shapes of the calculated and measured dielectric function spectra are in good agreement. The transitions in the Brillouin zone edge from the topmost and the second topmost VBs to the lowest CB are responsible for the main absorption peaks. However, also the energetically lower VBs contribute significantly to the high absorption coefficient.CTS and its related alloys are explored and investigated. For a perfectly crystalline CTS, reported experimental double absorption onset in dielectric function is for the first time confirmed by our calculations. We also found that the band gap energies of CTGS and CTSS vary almost linearly with composition over the entire range of x. Moreover, those alloys have comparable absorption coefficients with CZTSSe. Cu2XSnS4 (X = Be, Mg, Ca, Mn, Fe, Ni, and Zn) are also studied, revealing rather similar crystalline, electronic, and optical properties. Despite difficulties to avoid high concentration of anti-site pairs disordering in all compounds, the concentration is reduced in Cu2BeSnS4 partly due to larger relaxation effects. CuSb(Se1–xTex)2 and CuBi(S1–xSex)2 are suggested as alternative ultra-thin film absorber materials. Their maximum efficiencies considering the Auger effect are ~25% even when the thicknesses of the materials are between 50 and 300 nm.
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| 9. |
- Chen, Rongzhen, et al.
(författare)
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High absorption coefficients of the CuSb(Se,Te)2 and CuBi(S,Se)2 alloys enable high efficient 100 nm thin-film photovoltaics
- 2017
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Ingår i: EPJ Photovoltaics. - : EDP Sciences. - 2105-0716.
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate that the band-gap energies Eg of CuSb(Se,Te)2 and CuBi(S,Se)2 can be optimized for high energy conversion in very thin photovoltaic devices, and that the alloys then exhibit excellent optical properties, especially for tellurium rich CuSb(Se1−xTex)2. This is explained by multi-valley band structure with flat energy dispersions, mainly due to the localized character of the Sb/Bi p-like conduction band states. Still the effective electron mass is reasonable small: mc ≈ 0.25m0 for CuSbTe2. The absorption coefficient α(ω) for CuSb(Se1−xTex)2 is at ħω = Eg + 1 eV as much as 5–7 times larger than α(ω) for traditional thin-film absorber materials. Auger recombination does limit the efficiency if the carrier concentration becomes too high, and this effect needs to be suppressed. However with high absorptivity, the alloys can be utilized for extremely thin inorganic solar cells with the maximum efficiency ηmax ≈ 25% even for film thicknesses d ≈ 50–150 nm, and the efficiency increases to ~30%if the Auger effect is diminished.
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| 10. |
- Crovetto, Andrea, et al.
(författare)
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Dielectric function and double absorption onset of monoclinic Cu2SnS3 : origin of experimental features explained by first-principles calculations
- 2016
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier. - 0927-0248 .- 1879-3398. ; 154, s. 121-129
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we determine experimentally the dielectric function of monoclinic Cu2SnS3 (CTS) by spectroscopic ellipsometry from 0.7 to 5.9 eV. An experimental approach is proposed to overcome the challenges of extracting the dielectric function of Cu2SnS3 when grown on a glass/Mo substrate, as relevant for photovoltaic applications. The ellipsometry measurement reveals a double absorption onset at 0.91 eV and 0.99 eV. Importantly, we demonstrate that calculation within the density functional theory (DFT) confirms this double onset only when a very dense k-mesh is used to reveal fine details in the electronic structure, and this can explain why it has not been reported in earlier calculated spectra. We can now show that the double onset originates from optical transitions at the Gamma-point from three energetically close-lying valence bands to a single conduction band. Thus, structural imperfection, like secondary phases, is not needed to explain such an absorption spectrum. Finally, we show that the absorption coefficient of CTS is particularly large in the near-band gap spectral region when compared to similar photovoltaic materials. (C) 2016 Elsevier B.V. All rights reserved.
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